archive mirror
 help / color / mirror / Atom feed
From: Mike Snitzer <>
To: Nir Soffer <>
Cc: David Teigland <>,
	Vojtech Juranek <>,
	Denis Chaplygin <>,
Subject: Re: [linux-lvm] Mixing devices with different logical or physical block size in oVirt LVM based storage
Date: Mon, 4 Feb 2019 11:25:27 -0500	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Sat, Feb 02 2019 at  6:54pm -0500,
Nir Soffer <> wrote:

>    We working on enabling 4k block size in oVirt block storage domain, built
>    using VG
>    on multipath devices on shared storage.
>    We have incomplete support for 4k, added in 2011, for this bug:
>        [1]
>    When creating or extending a VG, we check that all PVs are using same
>    logical and
>    phyisical block size, and we store both logical and physical block size in
>    the VG tags.
>    We get the block sizes from
>    /sys/block/dm-X/queue/{logical,physical}_block_size.
>    We also enforce that device physical block size is not smaller than
>    logical block size,
>    This check was added in this patch, trying to enable block size != 512.
>    There is no
>    explanation in the patch or in the review comments why we need to validate
>    this.
>    [2]
>    When we start to use a VG, we validate that all the devices are using the
>    stored logical
>    and physical block size.
>    In vdsm itself, we use the logical block size to manage vdsm metadata,
>    assuming that writing
>    and reading one block of logical block size bytes is atomic, and we can
>    read and write
>    different blocks from different hosts at the same time.
>    The relevant code validating PV block sizes is here:
>    [3]
>    Reading the comments in bug 732980, I don't see anything about physical
>    block size. It looks
>    like this is unnecessary check, and we should check only the logical block
>    size.
>    Regarding mixing devices with different logical block size, according to
>        [4]
>    We should not extend an LV over devices with different block size, as this
>    will change the device
>    logical block size (e.g change from 512 to 4k), and the change may break
>    the upper layer that
>    already use the device and assume the previous logical block size.

This idea that 4K writes to a 512b physical drive aren't going to be
atomic, and that that is going to be the basis for some upper level
failure is handwaving and overly paranoid TBH.

>    Based on this, I think we are ok with limiting VG to devices with same
>    logical block size, so any
>    LV can be extended to any device.
>    I think this code should change to:
>    1. When creating a VG, check that all PVs use the same logical block size
>    2. Store the logical block size in the VG tag
>    3. When extending the VG, check that the new PVs use the same logical
>    block size
>    4. When starting to use a VG, check that stored logical block size matches
>    PVs logical block size
>    What do you think?

I think you shouldn't care.  Or please show me a case where all this
concern matters.


  reply	other threads:[~2019-02-04 16:25 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-02 23:54 [linux-lvm] Mixing devices with different logical or physical block size in oVirt LVM based storage Nir Soffer
2019-02-04 16:25 ` Mike Snitzer [this message]
2019-02-13  9:14   ` Vojtech Juranek
2019-02-13 20:39     ` Mike Snitzer
2019-02-13 21:41       ` Nir Soffer

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).